Positive ion source



April 1965 K. EKLUND 3,178,604

POSITIVE ION SOURCE Filed April 10, 1962 2 Sheets-Sheet 1 r 'INVENTOR. KARL. EKLUND ATTORNEYS April 13, 1965 K. EKLUND 3,178,604

POSITIVE ION SOURCE Filed April 10, 1962 2 Sheets-Sheet 2 INVENTOR KARL. EKLUND ATTORNEYS United States Patent York Filed Apr. 10, 1962, Ser. No. 186,487 8 Claims. (Cl. 313-147) The present invention relates to ion sources and, more particularly, to an ion source of high efficiency and long life.

The ion source of the present invention falls in that class of ion sources employing both electrostatic and magnetic focusing of an electron beam in a region containing an ionizable gas at relatively low pressures. The apparatus is provided with a filamentary cathode and an anode with an electrode disposed therebetween for electrostatically concentrating the electron beam in order to increase ionization efficiency in the gas. The concentration of the beam is further enhanced by utilization of electromagnets for producing a magnetic field between the electrode and the anode.

In the prior art apparatus, the electromagnetic structures were mounted internally of the source so that the coils of the electromagnets were subject to extreme heating during out-gassing of the source. This required the electromagnets to employ high temperature insulation increasing the cost of the device and the size of the electromagnets. Also, since the magnets are located internally of the source, they block the view of the discharge area and also prohibit air circulation about the discharge region necessitating liquid cooling of the structure.

Another difliculty with the prior art ion sources of the type set forth above results from the fact that the various electrodes are rigidly mounted relative to one another and difficulty is experienced in obtaining accurate alignment of the apertures through the electrodes. Since the elements are all rigidly mounted, alignment of the apertures must result from manufacture of the elements to extremely close tolerances which greatly increases the cost of fabrication of the units.

In accordance with the present invention, there is provided an ion source employing electrostatic and electromagnetic focusing of a plasma-generating, electron beam wherein the electromagnets are detachably mounted externally of the source so that the magnets may be removed during out-gassing of the apparatus. These electromagnets do not require high temperature insulation, are relatively inexpensive to manufacture, and are quite compact. In addition, location of the magnets externally of the device, in conjunction with the utilization of glass walls, permits the user to view the discharge area. Further, the electromagnets do not prevent flow of air directly over the discharge vessel and the device may be and is operated as an air cooled unit.

The unit of the present invention employs a construction permitting adjustment of the center lines of the apertures in the various members relative to one another after assembly of the device thereby reducing the cost of fabrication of the unit since the effects of inaccuracies in the initial fabrication of the various elements may be corrected by a simple adjustment of machine screws located externally of the device. The various members may be aligned after assembly and during operation of the device by maximizing the ion current output of the unit as determined by conventional instrumentation.

It is one object of the present invention to provide an iOn source employing electrostatic and electromagnetic focusing of a plasma-producing electron beam in which the magnets employed to produce the magnetic focusing field are located externally of and are removable from the ion source.

It is another object of the present invention to provide an ion source in which the apertures through various electrodes of the apparatus may be adjusted after assembly of the apparatus and during operation thereof by means of members located externally of the device.

It is still another object of the present invention to provide an easily assembled ion source employing electrostatic and magnetic focusing of a plasma-producing, elec tron beam wherein the focusing magnets may be removed from the structure during out-gassing of the unit.

It is yet another object of the present invention to provide an ion source employing electrostatic and electromagnetic focusing of a plasma-producing electron beam in which the discharge region may be viewed from externally of the source and during operation thereof.

Still another objectof the present invention is to provide an ion source employing electrostatic and magnetic focusing of a plasma-producing, electron beam in which the internal structure of the apparatus is sufiiciently uncluttered to permit the unit to be air-cooled rather than water-cooled.

It is yet another object of the present invention to provide an ion source employing electrostatic and magnetic focusing of a plasma-producing electron beam in which the beam focusing and concentrating electrodes have replaceable inserts located in the regions of high temperature so that these portions of the electrodes may be readily replaced, thereby increasing the life-of the apparatus.

The above and still further objects, features and advantag'es of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is an end view of the apparatus of the present invention;

FIGURE 2 is a cross-sectional view taken along section line 22 of FIGURE 1;

FIGURE 3 is a cross-sectional view taken along line 33 of FIGURE 2; and

FIGURE 4 is a detailedcross-sectional view of the electrode structures employed in the apparatus of the present invention.

Referring specifically to FIGURES 1 through 3 of the accompanying drawings, the ion source of the present invention comprises a centrally apertured, generally square, end plate 1 supporting a filament assembly genererally designated by the reference numeral 2. The filament assembly comprises a ribbon-type filament 3 supported on a circular plate 4 attached to the left surface of the plate 1 as viewed in FIGURE 1. The filament 3 is insulated from the filament plate 4 by means of lead-thru insulators 6 only one of which is illustrated in FIGURE 2. Wires 7 extend through the insulators 6 and are connected to the ribbon-like filament 3. Electrical connections are made to the wire 7 in order to heat the filament. The filament 3 is positioned internally of the ion source and extends through a large central aperture 8 in the end plate 1.

The plate 4 is further provided with an aperture 9 through which a neutral gas may be emitted to the interior of the apparatus at a controlled pressure, the gas being the source of ions of the apparatus. The plates 1 and 4 are fabricated from aluminum for purposes to be explained subsequently. The filament 3 may be coated with suitable electron emissive materials, these being well known in the art and forming no part in the present invention.

The left surface of the end plate 1 is provided with an annular groove 11 adapted to receive an O-ring 12 to provide an air-tight seal between the plates 1 and 4. The plate 4 is bolted to the outer surface of the plate 1, by a plurality of machine bolts 13, pressing against the seal 12 to provide the necessary air-tight engagement between the various members. In order to minimize the formation of organic vapors due to bombardment of the various O-rings by charged particles, they may be fabricated from indium or other metals rather than elastomeric materials.

Parallel to but spaced from the end plate 1 is a metal plate 14 having disposed in a centrally located aperture a conical electrode 16 constituting an element of the electrostatic and magnetic lens structure. The apex of the conical member 16 is apertured as at 21 with the apex being located along the center line of the apparatus and extending in a direction away from the filament 3. Disposed between the plate 1 and the plate 14 is a hollow annular glass or related member 17, the end surfaces of which mate with O-rings 18 and 19 disposed in grooves in adjacent faces of the plates 1 and 14.

A further plate 22 is spaced from and parallel to the plate 14 and is separated therefrom by means of a hollow glass cylinder 23. The cylinder 23 contacts O-rings 24 and 26 located in adjacent surfaces of the plates 14 and 22 so as to provide a seal between the member 23 and the members 14 and 22, respectively. The plate 22 is also centrally apertured and supports an anode structure generally designated by the reference numeral 27. The anode structure and that of the electrode 16 are described in detail subsequently.

A further plate 28 is spaced from and generally parallel to the plate 22 and is also centrally apertured. A hollow cylindrical glass member 29 extends between the plates 22 and 28 and contacts O-rings 31 and 32 seated in annular grooves respectively in the aforesaid plates. The plate 28 carries an extractor assembly 33 comprising a cylindrical adapter 34 disposed in the central aperture of plate 28 and connected thereto. An extractor 36, which is generally conical, has its apex directed toward the apex of the electrode 21, the apex being centrally apertured as at 37.

The ion source comprises two further plates 38 and 39 parallel to one another and to the plates 1, 14, 22 and 28. The plates 38 and 39 are centrally apertured and have disposed in the apertures and connected thereto electrodes 41 and 42. The electrode 41 is a focusing electrode for the ion beam and the electrode 42 serves as a drift compensator also for the ion beam.

The entire mechanism described above is placed under compression by four non-conducting tie-rods or cinchrods 43 which extend through aligned apertures located adjacent the corners in each of the plates 1, 14, 22, 28 and 38 and are held in apertures formed in the plate 39 by threaded engagement therewith or by other suitable means for instance, keyed bushings. Compression springs 44 situated on the left ends of the rods 43 are disposed between washers 46 and 47 with the washers 46 engaging the outer or left surface of the plate 1. Adjustment nuts 48, in threaded engagement with the left ends of each of the cinch rods, adjust the compression in the springs 44 and the nut 48 is locked into position by a further lock nut 49. The compressional forces exerted by the rods on the assembly must be suflicient to insure an air-tight seal between the mating surfaces of the various plates and glass cylinders through the O-ring seals.

The plates 1, 14, 22, 28, 38 and 39 are generally of the same planar dimensions and the corners of each of the aforesaid plates is cut at a generally 45 angle relative to each of the adjacent sides as indicated by reference numerals 51. Four elongated, generally rectangular nonconducting rods 52 extend between the plates 1 and 39 adjacent the surfaces 51 of the plates. The rods are securedto' the plates 1, 22, 38 and 39 as by machine bolts 53 and are spaced from these plates by spacers 55. Each of the bars 52 is provided with one each of machine bolts 54 and 56 which extend into abutting relation with surfaces 51 of the plates 14 and 28, respectively. The apertures in all of the plates 1, 14, 22, 28, 38 and 39 are slightly larger than the outer diameter of the rods 43 so that the plates may be moved relative thereto. All restraint of the plates relative to the rods is effected by centering rods 52. The four adjustment bolts 54 associated with the plate 14 and the four bolts 56 associated with the plate 28 are adjusted so as to accurately align the centers of the apertures 21 and 37 of the electrodes 16 and 36, respectively, with the centerline of the aperture of the anode structure 27 to be described subsequently.

Accurate alignment of these members is effected by measuring the ion current output from the device and ad justing the screws 54 and 56 until the output current is maximized. Since the entire stack of plates is held in position under compression due to the arrangement of tie-rods 43 and springs 44, the plates 14 and 28 are readily movable relative to the other members of the assembly while the air-tight seal is maintained due to the compressive forces provided by the tie-rods. Thus, the ion source may readily be adjusted for maximum output while in operation, this feature being very important in that it reduces the extreme accuracies to which the various parts must be machined during fabrication of the prior art ion sources. Once the apparatus has beenadjusted for maximum output, the adjustment screws 54 and 56 may be held in place by lock nuts 57 and 58, respectively;

Electrical connections to the various plates such as 14, 22, 28, 38 and 39 are effected by conventional means which form no part of the present invention and are not illustrated. v I

The magnetic structure, generally designated by the reference numeral 63, comprises an electrical coil 64 wound on a rectangular core 66. The core is supported between plates 67 and 68 bolted to an edge of the plates 1 and 22, respectively. Two such magnetic arrangements are provided and they are bolted to opposite sides of the aforesaid plates. The plates 14 and 22 and their associated electrodes 16 and 27 are fabricated from magnetic mate rial while the plates 1 and 4 are fabricated from aluminum, copper or other material having approximately the same magnetic permeability as air. A further ree tangular plate 69 extends perpendicular to the plate 68 and terminates adjacent the outer edge of the plate 14. Thus, a magnetic circuit is established from the plate 22, the plate 67, through the core 66 of the electromagnetic structure, the plates 68 and 69 to the plate electrode 14. The magnetic circuit has a small air gap between the electrode 16 and the anode assembly 27 and a high density mangetic field is established in the gap. I

In order to maintain the plates 14 and 22 electrically insulated from one another, there is provided a small in'' sulating spacer 71 between the members 67 and 22, 68 and 1, 69 and 14. The spacer introduces an air gap into the magnetic structure but it is quite small and does not appreciably aifect the reluctance of the magnetic path. The insulated spacer 71 is magnified relative to the remainder of the structure as illustrated in FIGURE 2 merely for purposes of clarity of illustration. The bolts which are employed to secure the plates 67, 68 to the plates 22 and 1, respectively, are non-conducting so as to prevent an electrical short circuit in the system.

It is readily apparent that the electromagnetic structure is arranged wholly externally of the apparatus and when it is desired to out-gas the unit, the bolts holding the plates 67 and 68 may be removed so that the electric coils 64 are not subject to high temperatures and need not be fabricated from expensive and high-temperature insulating materials.

Referring now specifically to FIGURE 4 of the accompanying drawings, the structures of the electrode 16, anode assembly 27 and extractor electrode 36 are illustrated in greater detail. It will be noted that the plate 14 is recessed as at 72 to provide an annular flange 73 terminating at a central aperture 74 of the plate 14. The electrode structure 16 is provided with an outer circular flange 76 which mates with the circular flange 73 in the plate 14 and the flange 76 is bolted to the flange 73. A conical portion 77 of the electrode 16 extends into the aperture 74 in the plate 14 and has a central aperture 21 at its apex located adjacent the anode structure 27.

The anode structure 27 comprises a plate 78 seated in a circular recess 79 in the plate 22, the recess formed in the surface of the plate 22 facing the electrode structure 16. The plate 78 is suitably bolted to the plate 22 and has bolted thereto a further member 81. The member 78 is provided with a centrally located, stepped recess 82 to provide a stepped aperture for receiving an anode insert 83 having a very small central aperture 85. The plate 81 extends across a portion of the anode insert 83 and therefore holds the anode insert 83 in the recess 82 formed in the plate 78, the plate 81 being bolted to the plate 78.

It is seen from the above that the elements subject to the highest heat and charged particle bombardment; namely, electrode 16 and anode insert 83, may readily be removed from their plates 14 and 22 and replaced. If no damage has been done to any of the portions of the anode assembly 23 other than the anode 83, the entire anode structure may be disassembled and the insert 83 replaced.

Referring now specifically to the extractor assembly, the extractor adapter 34 is provided with a circumferential recess 84 in its inner periphery and is adapted to receive a circular and axially extending extension 86 of the extractor 36. The extractor slips into the extractor adapter 34 and is snugly received therein, the extractor 35 and extractor adapter 34 abutting along their outer peripheries along the line 87. Thus, all of the members which are normally subject to high heat and a high intensity discharge and the erosion incident thereto are readily replaceable so as to prolong the life of the apparatus.

In operation of the apparatus of the invention, the electrode 16 is maintained at a potential positive relative to the plate 1 and negative relative to the plate 22. The electrons emitted by the filament 3 are accelerated toward the anode by the electrostatic field formed between the filament 3 and the electrode 16. The electrons pass through the apertures in the member 77 and are attracted to the anode 27. The electrostatic field between the electrode 16 and anode 27 is concentrated at the center of the device as a result of the conical slope of the member 16. The magnetic field established by the electromagnetic assembly 63 is also concentrated by the shape of the member 16 along the center line of the apparatus and the effect of the electrostatic and magnetic fields is to concentrate the electrons in a stream along the center line of the apparatus between the electrode 16 and the anode insert 83. The ionization of the neutral gas in this region results in the formation of a relatively neutral plasma which contains large quantities of the ion of the neutral gas. The extractor electrode 36 is operated at a highly negative potential relative to the anode assembly 27 and therefore the positive ions appearing in the region of the anode are accelerated along the centerline of the apparatus in the general direction of the extractor 36. Due to the fact that the aperture 85 of the anode structure is quite small whereas the adjacent aperture of the extractor 36 is quite large and due to the high intensity electrostatic field developed between the anode and the extractor, the positive ions are accelerated rapidly through the aperture of the extractor 3:) and are caused to proceed generally along the center line of the apparatus where they are subsequently concentrated by the electrodes 41 and 42.

It will be noted that due to the general assembly of the apparatus; that is, the use of the plurality of metallic plates and hollow glass cylinders which are held together by a cinch rod and spring assembly, the apparatus may be easily assembled. Any necessary adjustments are related wholly to positioning of the members 16 and 36 and, as indicated above, these may be accomplished by means of the adjustment screws 54 and 56 after the apparatus has been out-gassed and is operatmg.

Since all of the containing walls of the apparatus are made of glass or other suitable transparent material, the entire discharge area may be viewed by the operator greatly facilitating adjustment of the various members and permitting ready detection of certain dilficulties in this area which are visually observable. Also, this structure permits observation of any material Wear of the various members in the electrode structure including the filamentary cathode 3.

While I have described and illustrated one specific embodiment of my invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

What I claim is:

1. An ion source comprising a plurality of spaced and axially aligned, and centrally apertured conductive plates, a plurality of axially aligned, hollow, cylindrical, electrical insulating members disposed in alternating relation with and butting said plates thereby defining a hollow, cylindrical region, means for maintaining an air-tight seal between said plates and said cylindrical members, a first of said plates supporting a heatable filament in said region for producing electrons therein, a second of said plates supporting a conical electrode having its apex axially apertured and directed away from said filament, a third of said plates supporting an axially apertured anode adjacent the apex of said conical electrode, said second and third plates being paramagnetic, a magnetic structure removably supported externally of said region and secured to at least one of said paramagnetic plates, said magnetic structure providing a magnetic circuit between said second and third plates, means for establishing a magnetic flux in said magnetic circuit, and means for admitting a neutral gas into the region between said anode and said filament.

2. The combination according to claim 1 wherein said second plate is translatable in a plane perpendicular to the axis of said cylindrical region.

3. The combination according to claim 1 wherein said hollow, cylindrical members are transparent.

4. The combination according to claim 1 further comprising a conical extractor supported in a fourth of said plurality of plates, said conical extractor being centrally apertured and having its apex directed toward said anode and the apex of said conical electrode, and means for moving said fourth plate in a plane perpendicular to the axis of said cylindrical region.

5. The combination according to claim 1 wherein said means for establishing a magnetic flux comprises an electromagnet having a coil and an armature, means for supporting said armature at least on one of said second and third of said plurality of plates, and paramagnetic members extending from opposite ends of said armature to adjacent said second and third plates.

6. In an ion source, a plurality of generally parallel, axially aligned, and centrally apertured conductive plates, a plurality of hollow, cylindrical insulating members arranged in alternation with and in abutting relation with said plates to provide an axially extending assembly defining a hollow, generally cylindrical region, means for placing said assembly in compression, said means comprising a plurality of tie-rods extending through aligned apertures in said plates located externally of said region, the apertures formed in a second and fourth of said plates being sufliciently large relative to the external diameter of said tie-rods to permit movement of said second and fourth plates in a plane perpendicular to the axis of said region, the apertures in the remainder of said plates snugly receiving said tie-rods, and means for selectively moving said second and fourth plates.

7. The combination according to claim 6 wherein and further comprising a first plate supports a heatable filament internally of said region, a conical electrode removably secured to said second of said plurality of plates and having an apertured apex coaxial with the axis of said region and directed away from said filament, an apertured anode structured removably secured to a third of said plurality of plates and located adjacent the apex of said conical electrode, an extractor electrode removably secured to said fourth of said plurality of plates adjacent to said anode and on the opposite side thereof from said conical electrode, said second and third plates being paramagnetic and means detachably secured to said assembly externally of said region for establishing a magnetic circuit including said second and third plates.

8. The combination according to claim 7 wherein said means for selectively moving said second and fourth plates comprises a plurality of rods extending parallel to said axis of said region and arranged about said plates, said bars being secured to at least said first and third plates and adjustment bolts threaded into and extending through said bars and into engagement with said second and fourth plates.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN ION SOURCE COMPRISING A PLURALITY OF SPACED AND AXIALLY ALIGNED, AND CENTRALLY APERTURED CONDUCTIVE PLATES, A PLURALITY OF AXIALY ALIGNED, HOLLOW, CYLINDRICAL, ELECTRICAL INSULATING MEMBERS DISPOSED IN ALTERNATING RELATION WITH AND BUTTING SAID PLATES THEREBY DEFINING A HOLLOW, CYLINDRICAL REGION, MEANS FOR MAINTAINING AN AIR-TIGHT SEAL BETWEEN SAID PLATES AND SAID CYLINDRICAL MEMBERS, A FIRST OF SAID PLATES SUPPORTING A HEATABLE FILAMENT IN SAID REGION FOR PRODUCING ELECTRONS THEREIN, A SECOND OF SAID PLATES SUPPORTING A CONICAL ELECTRODE HAVING ITS APEX AXIALLY APERTURED AND DIRECTED AWAY FROM SAID FILAMENT, A THIRD OF SAID PLATES SUPPORTING AN AXIALLY APERTURED ANODE ADJACENT THE APEX OF SAID CONICAL ELECTRODE, SAID SECOND AND THIRD PLATES BEING PARAMAGNETIC, A MAGNETIC STRUCTURE REMOVABLY SUPPORTED EXTERNALLY OF SAID REGION AND SECURED TO AT LEAST ON OF SID PARAMAGNETIC PLATES, SAID MAGNETIC STRUCTURE PROVIDING A MAGNETIC CIRCUIT BETWEEN SAID SECOND AND THIRD PLATES, MEANS FOR ESTABLISHING A MAGNETIC FLUX IN SAID MAGNETIC CIRCUIT, AND MEANS FOR ADMITTING A NEUTRAL GAS INTO THE REGION BETWEEN SAID ANODE OF SAID FILAMENT. 